Tagging of articles for identification and/or theft protection is known. For instance, many articles are identified using a bar code comprising coded information which is read by passing the bar code within view of a scanner. Many articles also include a resonant tag for use in theft detection and prevention. More recently, passive resonant security tags which return unique or semi-unique identification codes have been developed. These security tags typically include an integrated circuit which stores the identification code. Such "intelligent" security tags provide information about an article to which the tag is affixed which is detected in the zone of an interrogator. The tags are desirable because they can be interrogated rapidly, and from a distance. U.S. Pat. No. 5,446,447 (Carney et al.), U.S. Pat. No. 5,430,441 (Bickley et al.), and U.S. Pat. No. 5,347,263 (Carroll et al.) disclose three examples of intelligent tags.
Intelligent tagging of articles provides substantial benefits at the point of manufacture, at the point of distribution, and at the point of sale. That is, any place where articles are stored, shelved, displayed or inventoried, intelligent tags can result in substantial cost savings. For example, one function of a distribution center is to take merchandise that has been packed and shipped in bulk, and repack the merchandise into smaller "tote" boxes. Often the tote box is packed with single units of a variety of products. Mistakes in inventory during this repacking process can be very costly and there is a possibility of shipping products to the wrong retailer. An intelligent tagging system can check the contents of tote boxes with an interrogator or point reader at high speeds and confirm exactly what is being shipped to individual retailers.
Employees today spend many hours hand counting articles for inventory control and manually checking product expiration dates. Intelligent tags obviate the need for such hand counting and manual data checking. Rather than hand counting a plurality of items, an employee can point an intelligent tag reader at individual product clusters on shelves and scan entire product groups in minutes. Intelligent tags also allow employees to scan a product group to learn critical expiration dates to avoid spoilage, reduce stock and maintain continuous inventory counts.
Another example of an environment in which the use of intelligent tags is desirable is a library. Manual taking of inventory of a library collection is an expensive and time consuming task. Currently inventory taking is such an expensive and time consuming task that most libraries do not conduct a full inventory check as frequently as they should, if at all. Accordingly, there is a need for systems which allow library employees to efficiently check their inventory. Intelligent tags fulfill such a need.
One problem with attempting to read multiple RFID tags within an interrogation zone of a reader is that more than one tag could be activated by the reader or interrogator at about the same time, such that two or more tags may transmit their identification information to the reader at about the same time, thus causing the information to collide, which corrupts the information and prevents the reader from obtaining the desired information. In order to overcome such data collisions, some interrogators include a means for controlling the transmission of data from individual tags, for example, by shutting individual tags off for predetermined time periods after a response signal is transmitted. However, the transmission of a signal by the interrogator to an individual tag to shut the tag off may require the generation of signals in excess of the levels allowed by regulatory authorities, such as the Federal Communications Commission (FCC). Other systems include tags which include circuitry to detect the simultaneous transmission of data by multiple tags. Upon detection of such simultaneous transmissions, the tags abort their transmissions and wait for a prescribed time prior to retransmission, usually for a period of time that is set by a random number. However, this method requires the tags include detection circuitry and a battery, both of which excessively increase the cost of the tag. Accordingly, there is a need for a method of detecting substantially simultaneous transmission of data by multiple tags at the same frequency located within an interrogation zone and compensating for such multiple transmissions in order to accurately read the data transmitted by each tag.
The present invention provides a method of simultaneously reading multiple RFID tags located in a field of an interrogating antenna based on periodic transmissions from the tags with long non-transmission intervals between transmissions. The non-transmission intervals are fixed for a given tag, but are random between tags, preferably due to manufacturing tolerances, such that no co-ordination of transmissions from the interrogating antenna is required.